ABSTRACT: Genesis of Clay Mineral Assemblages and Micropaleoclimatic Implications in the Tertiary Powder River Basin, Wyoming

R. M. Flores, D. Bossiroy, J. N. Weaver, J. Thorez

An x-ray diffraction (XRD) study was undertaken on the clay mineralogy of the early Tertiary coal-bearing sequences of the Powder River basin. The vertical and lateral distribution of alternating fluvial conglomerates, sandstones, mudstones, shales, coals, and paleosols reveals a transition from alluvial fans along the basin margin to an alluvial plain and peat bogs basinward. Samples included unweathered shales and mudstones from a borehole and a variety of corresponding surface outcrop samples of Cambrian to Eocene age. Samples older than Tertiary were collected along the basin margin specifically to determine the potential source of parent material during Tertiary sedimentation.

XRD analyses were performed on the <2-µm fraction prepared as oriented aggregates. To investigate the materials in their natural state, no chemical pre-treatments we applied before the analysis. A series of specific post-treatments, consisting of catonic saturation (Li+, K+), a solution with polyalcohols, heating, acid attack, and hydrazine saturation, was selectively applied. These post-treatments permit a good discrimination between the mimetic clay minerals such as smectite and illite-smectite mixed layers that constitute the bulk of the clay fraction in the Tertiary rocks. When analyzed only using routine XRD, these swelling minerals are apparently uniformly distributed in the fluvial sedimentary rocks and are better interpreted as a single smectitic population. However, t e post-treatments clearly differentiate both qualitatively and quantitatively this smectitic stock. Other clays include illite and kaolinite, which have different degrees of crystallinity, and minor interstratified clays (i.e., illite-chlorite, chlorite-smectite). The clay minerals in pre-Tertiary (and pedogenic) materials are different from those in the Tertiary rocks. The smectitic population in the Tertiary rocks is distinguished into species on the basis of crystallochemical (composition, crystallinities) and genetic (reworked and inherited, synsedimentary, in-situ neoformed, volcanogenic or pedogenetic derived) properties.

Paleosols are marked by a well-crystallized kaolinite, when associated with sedimentary rocks underlying coal, or by a neoformed beidellite, in the case of climatically controlled vertisols. These clay minerals are superimposed on the inherited (primary detrital) clay assemblages. Such a mineralogic discrimination provides information about the prevailing micropaleoclimatic conditions that have existed during the deposition (cf. the clay composition of the paleosols developed under permanently wet or during alternating wet and dry conditions) and permit distinction between volcanogenic vs. inherited and pedogenic input.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990